Prometheus plugin

Provides a Prometheus exporter to pass on Ceph performance counters from the collection point in ceph-mgr. Ceph-mgr receives MMgrReport messages from all MgrClient processes (mons and OSDs, for instance) with performance counter schema data and actual counter data, and keeps a circular buffer of the last N samples. This plugin creates an HTTP endpoint (like all Prometheus exporters) and retrieves the latest sample of every counter when polled (or “scraped” in Prometheus terminology). The HTTP path and query parameters are ignored; all extant counters for all reporting entities are returned in text exposition format. (See the Prometheus documentation.)

Enabling prometheus output

The prometheus module is enabled with:

ceph mgr module enable prometheus

Configuration

By default the module will accept HTTP requests on port 9283 on all IPv4 and IPv6 addresses on the host. The port and listen address are both configurable with ceph config-key set, with keys mgr/prometheus/server_addr and mgr/prometheus/server_port. This port is registered with Prometheus’s registry.

Statistic names and labels

The names of the stats are exactly as Ceph names them, with illegal characters ., - and :: translated to _, and ceph_ prefixed to all names.

All daemon statistics have a ceph_daemon label such as “osd.123” that identifies the type and ID of the daemon they come from. Some statistics can come from different types of daemon, so when querying e.g. an OSD’s RocksDB stats, you would probably want to filter on ceph_daemon starting with “osd” to avoid mixing in the monitor rocksdb stats.

The cluster statistics (i.e. those global to the Ceph cluster) have labels appropriate to what they report on. For example, metrics relating to pools have a pool_id label.

The long running averages that represent the histograms from core Ceph are represented by a pair of <name>_sum and <name>_count metrics. This is similar to how histograms are represented in Prometheus and they can also be treated similarly.

Pool and OSD metadata series

Special series are output to enable displaying and querying on certain metadata fields.

Pools have a ceph_pool_metadata field like this:

ceph_pool_metadata{pool_id="2",name="cephfs_metadata_a"} 1.0

OSDs have a ceph_osd_metadata field like this:

ceph_osd_metadata{cluster_addr="172.21.9.34:6802/19096",device_class="ssd",ceph_daemon="osd.0",public_addr="172.21.9.34:6801/19096",weight="1.0"} 1.0

Correlating drive statistics with node_exporter

The prometheus output from Ceph is designed to be used in conjunction with the generic host monitoring from the Prometheus node_exporter.

To enable correlation of Ceph OSD statistics with node_exporter’s drive statistics, special series are output like this:

ceph_disk_occupation{ceph_daemon="osd.0",device="sdd", exported_instance="myhost"}

To use this to get disk statistics by OSD ID, use either the and operator or the * operator in your prometheus query. All metadata metrics (like `` ceph_disk_occupation`` have the value 1 so they act neutral with *. Using * allows to use group_left and group_right grouping modifiers, so that the resulting metric has additional labels from one side of the query.

See the prometheus documentation for more information about constructing queries.

The goal is to run a query like

rate(node_disk_bytes_written[30s]) and on (device,instance) ceph_disk_occupation{ceph_daemon="osd.0"}

Out of the box the above query will not return any metrics since the instance labels of both metrics don’t match. The instance label of ceph_disk_occupation will be the currently active MGR node.

The following two section outline two approaches to remedy this.

Use label_replace

The label_replace function (cp. label_replace documentation) can add a label to, or alter a label of, a metric within a query.

To correlate an OSD and its disks write rate, the following query can be used:

label_replace(rate(node_disk_bytes_written[30s]), "exported_instance", "$1", "instance", "(.*):.*") and on (device,exported_instance) ceph_disk_occupation{ceph_daemon="osd.0"}

Configuring Prometheus server

honor_labels

To enable Ceph to output properly-labeled data relating to any host, use the honor_labels setting when adding the ceph-mgr endpoints to your prometheus configuration.

This allows Ceph to export the proper instance label without prometheus overwriting it. Without this setting, Prometheus applies an instance label that includes the hostname and port of the endpoint that the series came from. Because Ceph clusters have multiple manager daemons, this results in an instance label that changes spuriously when the active manager daemon changes.

If this is undesirable a custom instance label can be set in the Prometheus target configuration: you might wish to set it to the hostname of your first mgr daemon, or something completely arbitrary like “ceph_cluster”.

node_exporter hostname labels

Set your instance labels to match what appears in Ceph’s OSD metadata in the instance field. This is generally the short hostname of the node.

This is only necessary if you want to correlate Ceph stats with host stats, but you may find it useful to do it in all cases in case you want to do the correlation in the future.

Example configuration

This example shows a single node configuration running ceph-mgr and node_exporter on a server called senta04. Note that this requires to add the appropriate instance label to every node_exporter target individually.

This is just an example: there are other ways to configure prometheus scrape targets and label rewrite rules.

prometheus.yml

global:
  scrape_interval:     15s
  evaluation_interval: 15s

scrape_configs:
  - job_name: 'node'
    file_sd_configs:
      - files:
        - node_targets.yml
  - job_name: 'ceph'
    honor_labels: true
    file_sd_configs:
      - files:
        - ceph_targets.yml

ceph_targets.yml

[
    {
        "targets": [ "senta04.mydomain.com:9283" ],
        "labels": {}
    }
]

node_targets.yml

[
    {
        "targets": [ "senta04.mydomain.com:9100" ],
        "labels": {
            "instance": "senta04"
        }
    }
]

Notes

Counters and gauges are exported; currently histograms and long-running averages are not. It’s possible that Ceph’s 2-D histograms could be reduced to two separate 1-D histograms, and that long-running averages could be exported as Prometheus’ Summary type.

Timestamps, as with many Prometheus exporters, are established by the server’s scrape time (Prometheus expects that it is polling the actual counter process synchronously). It is possible to supply a timestamp along with the stat report, but the Prometheus team strongly advises against this. This means that timestamps will be delayed by an unpredictable amount; it’s not clear if this will be problematic, but it’s worth knowing about.